(a) Field of the invention:
The present invention relates to a zoom lens system, and more specifically to a zoom lens system for use with cameras using electronic image pickup tubes or solid-state image pickup devices as the image sensors.
(b) Description of the prior art:
Most of the lens systems for use with cameras using electronic image pickup tubes or solid-state image pickup devices as the image sensors are designed as zoom lens systems. These zoom lens systems are mostly specified for aperature ratios of at least F/2.8 and zooming ratios of at least 3. Though these zoom lens systems have high aperture ratios and high vari-focal ratios, they comprise lens elements in numbers as large as 13 to 15 and require high manufacturing costs.
Recently, there are known zoom lens systems of this type comprising 12 to 18 lens elements as exemplified by the lens systems disclosed by Japanese Unexamined Published Patent Applications No. 123817/60 and No. 247318/62. Out of these zoom lens systems, the zoon lens system disclosed by Japanese Unexamined Published Patent Application No. 247318/62 has a fundamental composition consisting, in the order from the object side, of a first lens unit which has positive refractive power, and is kept fixed for varying focal length but movable for focusing, a second lens unit which has negative refractive power and is movable for varying forcal length, a third lens unit which has positive refractive power and is moved along a locus convex on the image side for correcting the position of image surface deviated by varying focal length, and a fourth lens unit which has an imaging function and is always kept fixed. This zoom lens system comprises three lens elements in the first lens unit, three lens elements in the second lens unit, one lens element in the third lens unit and four lens elements in the fourth lens unit, i.e. 11 lens elements in total, and is one of the zoom lens systems currently known as those comprising small numbers of lens elements.
In these days, there is a trend to design video movie cameras and still video cameras which are compacter in external dimensions, lighter in weights and manufacturable at lower costs. Conventionally, photographic lens systems for use with these cameras were designed mostly for zooming ratio on the order of 6, aperture ratios on the order of F/1.4 and field angles on the order of 50.degree. at the wide position. However, these photographic lens systems are recently being replaced with zoom lens systems designed for lower specifications of zooming ratios of 2 to 3 and aperture ratios on the order of F/2.0 to F/2.8. When the specifications are lowered as described above, it is easy to obtain zoom lens systems which are compact in external dimensions, light in weight and manufacturable at low costs. However, there has been obtained no zoom lens system yet which is compact in external dimensions, light in weight and manufacturable at a low cost sufficiently paying the lowering of the specifications.
The zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 169611/63, for example, requires a high manufacturing cost due to the large diameter thereof, comprises three lens elements in the first lens unit thereof a consists of 10 lens elements in total. However, this zoom lens system has a long distance as measured from the first surface to the final surface thereof, and is not designed for reserving a back focal length sufficient for allowing to arrange a splitting mirror for a viewfinder and a optical low pass filter for preventing moire after the lens system. Furthermore, this zoom lens system has Petzval's sum of a negative value, produces remarkable flare due to coma, hardly allows to reserve sufficient thickness at the marginal portions of the lens elements, and has a tendency to make intensity of marginal rays insufficient.
In addition, this conventional example of zoom lens system has a field angle of 45.degree. at the wide position, a vari-focal ratio of 3 and aperture ratios of F/1.6 to F/2.1.